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  Using deep cycle battery as starting battery

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Author Topic:   Using deep cycle battery as starting battery
bloller posted 05-13-2012 05:18 PM ET (US)   Profile for bloller   Send Email to bloller  
I've got a five year old group 24 flooded cell starting battery that still seems to work fine but I'm thinking I should go ahead and replace it for safety. The only accessories I use are nightime running lights and a radio.

I've also got a nearly new group 27 deep cycle flooded cell battery that I used a few times for a trolling motor that I have since sold. Would it be okay to use this battery as my starting battery. The label indicates that it is good for starting and it actually has more Cold Cranking Amps than my old starting battery.

Tom Hemphill posted 05-13-2012 05:33 PM ET (US)     Profile for Tom Hemphill  Send Email to Tom Hemphill     
You've pretty much answered your own questions. A five year old battery on a boat should be replaced. It may go another year or two, but it's better to address it while on dry land. Deep cycle batteries are fine for starting, provided they have sufficient capacity.
jimh posted 05-13-2012 05:56 PM ET (US)     Profile for jimh  Send Email to jimh     
The engine starter motor does not care what type of battery supplies the current for cranking; it only wants the battery to supply enough current and to not have much voltage sag. If you have a battery that meets the specifications for your outboard engine electric starting function, it should work fine, no matter the type.

The principal difference between a starting-lighting-ignition (SLI) battery and a deep-cycle battery is the tolerance for being deeply discharged. A SLI battery does not tolerate being completely or deeply discharged too often. In the case of a five-year-old SLI battery, you probably are down to one discharge cycle left, that is, the next time it becomes deeply discharged it will be done.

A deep-cycle battery is made to be tolerant of deep-discharge and re-charge cycles. The rub is that usually a deep-cycle battery will be physically much larger than a SLI battery that can supply the same cranking current. If you have room for the larger battery size, you can use a deep-cycle battery for engine cranking, but it will probably be larger than an equivalent-rating in a SLI battery.

The cranking Ampere rating is usually proportional to the surface area of the lead plates in the battery. In an SLI battery the plates are usually thinner than plates in a deep-cycle battery. That is how the SLI battery can produce more cranking Amperes in a smaller size.

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